Fix fill tool only filling upwards sometimes
The fill algorithm was behaving erratically and sometimes only filling
in top parts of shapes.
With this commit, the kpFillLine is copied by value. That seems to
fix the issue, as the previously used reference was invalidated by the
QList resizing/reallocating in kpFloodFill::addLine.
It should be noted that i have practically no C++ experience and the assumption may be fundamentally incorrect.